BUG 2138 - Do not fail on module-based default shard
Currently, DistributedShardedDOMDataTree will try to create default
shards on its start. However, this can collide with module-based default
shards. If present in modules.conf, modules-based default shards will be
created on DistributedDatastore's start.
If already present, do no create default shards. Create just
DistributedShardFrontend for them.
Change-Id: I05857f520e3467116e8748e6ae231ab9dc39f44c
Signed-off-by: Jakub Morvay <jmorvay@cisco.com>